It is a nice frame but i would put it to the mid fifties. I have the identical frame in 58cm. Mine came as a complete bike but it was "upgraded" with later wheels, bars and stem and suntour shift gruppo....
The only thing that was original were the brakes consisting of Universal levers and MAFAC dural forge brakes. Not knowing better I changed to Universal sidpulls but will change back to the Mafacs. I have learned that this combination was used by most of the pros at that time. I am now looking for a right crankset, probably Magistronis. Shifting on mine is Campa period correct but Simplex would fit better. I am excited how yours will turn out. You can see pictures of mine here:
http://www.wooljersey.com/gallery/v/Drexl/Cinelli+B/